1. Field of the Invention
The present invention pertains to marine seismic surveying and, in particular, to towed-array surveys.
2. Description of the Related Art
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying acoustic source(s) and acoustic sensors at predetermined locations. The sources impart acoustic waves into the geological formations. The acoustic waves are sometime also referred to as “pressure waves” because of the way they propagate. Features of the geological formation reflect the pressure waves to the sensors. The sensors receive the reflected waves, which are detected, conditioned, and processed to generate seismic data. Analysis of the seismic data can then indicate probable locations of the hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. Note that marine surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. One type of marine survey may be referred to as a “towed-array” survey. In a towed-array survey, a survey vessel tows an array of seismic cables extending linearly from the stern of the survey vessel. Each seismic cable, or “streamer”, includes a variety of instruments that provide a number of functions. Some of these instruments, typically acoustic receivers called “hydrophones”, receive the reflected waves and generate data as described above.
In conventional towed-array surveys, the seismic cables are commonly towed some 50-100 meters apart. This design specification arises from the theory of discrete spatial sampling, which dictates the maximum cable spacing for array detection of broadside seismic signal and the discrimination and suppression of broadside linear noise. The maximum cable separation, in turn, imposes a limit on overall cross-line receiver array coverage, since the total number of seismic cables is constrained by the towing capacity of any given seismic vessel.
This limitation on cable separation has a number of important consequences. For instance, it limits the maximum spread of the array, which increases the number of passes the survey vessel and towed array must make to cover the survey array. This increases the cost of the survey. It also imposes technological constraints on the survey. For instance, it limits the angle of incidence for the acoustic waves both at reflectors in the subterranean geological formation as well as the receivers of the streamers.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.